The information provided below is not admitted to be prior art to the present invention, but is provided solely to assist the understanding of the reader.
A drawback to the use of initiators or photoinitiators is the production of volatile low molecular weight fragments that may not be environmentally safe.
These resins are characterized by the presence of acrylate groups as pendant moieties and by the ability of these resins to cure under standard UV-cure conditions to give tack-free coatings without the addition of traditional photoinitiators.
Multifunctional acrylates and methacrylates (“acrylates”) are commonly utilized in the preparation of crosslinked films, adhesives, foundry sand binders, composite structures, and other materials. Acrylate monomers and oligomers may be crosslinked by free radical chain mechanisms, which may require any of a number of free radical generating species, such as peroxides, hydroperoxides, or azo compounds, that may decompose to form radicals either when heated, or at ambient temperatures in presence of promoters.
An alternative means of initiating reaction is the use of ultraviolet (UV) light or electron beam (EB) radiation to decompose photoinitiators to free radicals. For numerous applications, this method offers the potential of extremely rapid processing because the transformation from a liquid reactive composition to a crosslinked solid is essentially instantaneous upon exposure to UV or EB radiation.
A drawback to the use of initiators to effect free radical reaction, is that the decomposition of initiators and photoinitiators produces low molecular weight fragments that may volatilize during and/or after the manufacturing process. Fugitive emissions create safety issues regarding workers, consumers, and the environment. For instance, these low molecular weight fragments tend to be readily absorbed through skin which can cause adverse health effects.
These limitations have been addressed in several key approaches. The challenge of fugitive emissions during manufacturing processes or subsequent leaching of photoinitiator fragments has been attacked by creating acrylate monomers/oligomers with “built-in” photoinitiators. This may be accomplished by starting with a compound which is known to function as a photoinitiator (or a suitable derivative) and either functionalizing it with an appropriate unsaturated group, i.e. acrylate or methacrylate, so as to produce a new compound which functions as both monomer/oligomer and photoinitiator, or by “grafting” onto a preformed oligomer/polymer in order to produce a higher molecular weight photoinitiator.
Regardless of the effectiveness of these methods, they add additional manufacturing procedures and costs.
Moreover, these approaches result in resins of low functionality. Low functionality is detrimental to reactivity and final properties, and may impose a requirement for external catalyst or initiator to effect crosslinking.
The photo-polymerizable units of the ionic, UV-curable resins of the present invention are provided by the Michael addition of β-dicarbonyl compounds to acrylate acceptors. The Michael addition of acetoacetate donor compounds to multi-functional acrylate receptor compounds to make crosslinked polymers has been described in the literature. For example, Mozner and Rheinberger reported the Michael addition of acetoacetates to triacrylates and tetracrylates. (Macromolecular Rapid Communications, 16, 135-138 (1995)). The products formed were crosslinked gels. In one such reaction, depicted in FIG. 1, Mozner added one mole of trimethylol propane triacrylate (TMPTA) having three functional groups to one mole of polyethylene glycol (600 molecular weight) diacetoacetate (PEG600-DAA) having two functional groups. (Each acetoacetate “functional group” reacts twice, thus each mole of diacetoacetate has four reactive equivalents.) The resulting network is considered “gelled”, or cured, despite the presence of unreacted acrylic functional groups. While further reaction can be promoted, this network cannot be made liquid either with heat or solvent because it is effectively crosslinked.
A more recent and effective solution is described in U.S. Pat. Nos. 5,945,489 and 6,025,410 to Moy et al and assigned to Ashland, Inc., the assignee of the present application. Such approach involves reacting multifunctional acrylates with acetoacetates via Michael Addition in ratios that yield uncrosslinked, acrylate-functional resins. These resins crosslink upon exposure to an appropriate UV source in the absence of added photoinitiators.
Ultraviolet (UV)-curable waterborne coatings are interesting because of their advantages of environmental protection, lower energy consumption, high curing speed, rheological control, and adaptation to spraying. Conventionally, curable aqueous dispersions are obtained by either extra-emulsification or self-emulsification. Self-emulsification of acrylate ionomers is achieved by introducing hydrophilic ionic groups into the backbone of curable resins. A balance between dispersibility and water resistance can be achieved by incorporating some polyethylene oxide segments into the backbone of the ionic curable resin.
There exists a need for water-dispersible, UV-curable resins that incorporate the advantages of self-photoinitiation common to Michael resins.